The first elongated bone (metacarpal) at the base of the thumb is connected to the wrist trapezium through the carpometacarpal (CMC) joint which is sometimes referred to as the trapeziometacarpal joint. This particular joint is of critical anatomical importance to humans, due to the opposing motion of the thumb with respect to the fingers for grasping objects and performing daily functions. Damage to the CMC joint through physical injury or disease can therefore be a severe physiological burden to inflicted humans.
Osteoarthritis, rheumatoid arthritis, and post-traumatic arthritis of the carpometacarpal (CMC) joint cause interminable pain and poor function of the thumb. Patients who have mild symptoms often respond to rest, immobilization, non-steroidal anti-inflammatory drugs, or intra-articular injections of steroids. However, patients who have more severe forms of trapeziometacarpal (carpometacarpal) arthritis may have dorsoradial subluxation of the joint, adduction contracture of the first web space, severe loss of articular cartilage, secondary metacarpophalangeal hyperextension and interphalangeal flexion deformity. Carpometacarpal arthritis is often an isolated entity, without significant concomitant involvement of the trapezioscaphoid, trapezio-trapezoid, or trapezium-first metacarpal joint. Moreover, retaining the trapezium in patients who have isolated trapeziometacarpal arthritis is justified in order to preserve stability at the base of the thumb (Lister, et al., "Arthritis of the Trapezial Articulations Treated by Prosthetic Hand", 9:117-129 (1977)). Carpometacarpal total joint replacement can restore the length and direction of the first metacarpal by reducing any existing subluxation of the metacarpal on the trapezium and by providing a fixed fulcrum for the metacarpal to articulate with the trapezium.
The goal of surgical intervention in patients who have CMC arthritis is to restore a strong, painless thumb without deformity or decreased motion. Many reconstructive procedures have been devised including interpositional arthroplasty, resection arthroplasty of the trapezium, resection interpositional arthroplasty of the trapezium, and arthrodesis (see, e.g. Ferrari, B. et al., The Journal of Bone and Joint Surgery, Vol. 68-A:8, 1117-1184 (1986)). Various types of prosthetic joints have been developed for surgical replacement of the metacarpophalangeal and interphalangeal joints of the fingers including the CMC joint of the thumb. Examples of prostheses are shown in the following U.S. Pat. Nos. 5,092,896, issued Mar. 3, 1992; 4,955,916, issued Sep. 11, 1990; 4,276,660, issued Jul. 7, 1981; 4,242,759, issued Jan. 6, 1981; 4,231,121, issued Nov. 4, 1980; and 3,924,276, issued Dec. 9, 1975. Similar implants have been developed for the toe as described in U.S. Pat. Nos. 5,037,440, issued Aug. 6, 1991; and 4,908,031, issued Mar. 13, 1990. These patents generally describe joint prostheses having concave and convex bearing areas and variations thereof which tend to be of conventional ball and socket structural design.
Although these aforementioned designs and procedures have been shown to benefit certain patients, none fulfill the requirements of an ideal arthroplasty which restores natural and unaffected thumb use (see, Menon, et al., "The Problem of Trapeziometacarpal Degenerative Arthritis", Arthritis. Clin. Orthop., 175:155-165 (1983)). Therefore, there is a need for a maintenance-free CMC joint replacement which restores to a patient having a damaged or diseased joint pain-free thumb use with natural dexterity and longevity of use.
Accordingly it is the object of the present invention to construct an improved CMC joint prosthesis which allows essentially original and natural function to be restored to the damaged thumb. These and other objects of the invention will be readily apparent from the following detailed description and the accompanying drawings.